This invention relates to the drying of foams in a manner to preserve internal cellular foam structure, and in particular to the drying of aerogels.
Materials consisting substantially of an air filled cellular network such as foam rubber, and "blown" plastic foams such as polyurethane and polystyrene, are common, everyday products. They are employed in great quantities for cushions in furniture, insulation for home refrigerators, etc. In recent years a new group of open-cell foams have been devised called "aerogels". Aerogels are derived from highly cross-linked inorganic or organic gels. Aerogels are characterized by an ultra-fine cell/pore size of approximately 1 to 10 nm. across, continuous porosity, with a microstructure composed of polymeric chains or colloidal like particles interconnected together.
This microstructure endows aerogels with unique properties including extremely low thermal conductivity, making them of interest for purposes of thermal insulation. For example, aerogels made from silica can be used for transparent window insulation. And low cost organic aerogels are of interest as a replacement for traditional fluorocarbon blown urethane foams used for refrigeration insulation, and the like.
In the manufacture of aerogels, typically a final stage is reached in which a diluent filled gel must be dried. Evaporative drying to remove this liquid diluent usually causes the delicate pore structure to collapse due to large capillary forces during the drying process. Similarly freeze drying will often damage the gel microstructure due to large solvent crystals at the freezing front. In order to preserve the gossamer structure of inorganic and particularly organic aerogels, critical point drying has been found the method of choice. In this drying technology the liquid diluent is usually replaced with a solvent such as alcohol or acetone, and this solvent is then replaced with a transitional fluid such as liquid carbon dioxide. The liquid carbon dioxide saturated gel is now placed in a pressure vessel, and the vessel is heated to bring the liquid carbon dioxide to its critical temperature and pressure. The now gaseous transitional fluid may now be bled off without surface tension distortion of the microporous structure of the aerogel.
In U.S. Pat. No. 4,735,794 I disclose a simpler method in comparison to the normally employed critical point specimen drying for preparing specimens for the scanning electron microscope. It is believed this method will yield similar benefits for optimum three-dimensional structural maintenance for porous foam drying in general, and in particular in maintaining microporous structure within inorganic and organic aerogels.
Accordingly it is an object to provide a simple method for preserving porous structure during the drying of foams.
Another object is to provide a method for maintaining microporous pore structure during the drying of aerogels.
A further object is to provide a method for preserving microporous structure in organic aerogels.
Still another object is to provide for large scale drying of aerogels in a practical manner.